In recent years, digital broadcasts that transfer information such as a voice, a picture, a character, or the like as a digital signal via a transfer path including ground waves or satellite waves have been further developed. One method for transferring a digital signal is a suggested by ISO/IEC13818-1. The ISO/IEC13818-1 describes a method for multiplexing and transferring an encoded digital signal including voice, picture, and piece of data of a program on a transmission side and receiving and reproducing of a specified program on a reception side.
The encoded voice signal and picture signal are divided by predetermined time and are provided with header information including, for example, reproduction time information, forming a packet called PES (Packetized Elementary Stream). The PES is basically divided in units of 184 bytes. The PES is additionally provided with header information including, for example, a packet identifier (PID), and is reconstructed to be a packet called a TSP (transport packet) to be multiplexed. Moreover, table information called PSI (Program Specific Information) indicating relationship between a program and a packet forming the program is multiplexed to the TSP of the voice signal or the picture signal. Defined as the PSI are four kinds of tables including a PAT (Program Association Table) and a PMT (Program Map Table). Described in the PAT is a PID of the PMT corresponding to each program, and described in the PMT is a PID of a packet in which, for example, a voice or picture signal forming the corresponding program is stored.
A receiver refers to the PAT and the PMT to thereby extract, from the TSP having a plurality of multiplexed programs, a packet forming a target program. The data packet and the PSI are stored into the TSP in a format called a section different from the PES. Extracting from the PES packet data excluding the header, etc. can provide, for example, an MPEG-2 AAC stream.
Before transferring a signal such as, for example, a voice signal to the receiving device, the signal may be encoded. As a method of encoding the voice signal, there is ISO/IEC 13818-7 (MPEG-2 Audio AAC). For the AAC standard used in the digital broadcast, the current service supports a 5.1 channel. For Japanese digital broadcasts, ARIB standards and operation specifications issued by Association of Radio Industries and Businesses are provided, which define in detail specifications of detailed methods, parameters, and operation.
FIG. 13 illustrates a table showing specified voice component types as defined by ARIB STD-B10. In a 2-channel stereo broadcast, a 2/0 mode (stereo) shown in this figure is typically used. In a surround broadcast, a 3/2+LFE mode is used to carry out a so-called 5.1-channel surround broadcast.
FIG. 14 illustrates a block diagram of a digital broadcast transmitting device 1400. The block diagram focuses on a function relating to switching between a 2-channel stereo broadcast and a surround broadcast. The digital broadcast transmitting device 1400 includes a sequence control unit 142, a voice signal input switching unit 150, a voice signal encoding unit 151, a packetizing unit 152, a descriptor encoding unit 153, a packetizing unit 154, a multiplexing unit 155, and a modulation unit 156.
An instruction for making switching manually or based on delivery programming is inputted to the sequence control unit 142. The sequence control unit 142, defining a switching point, controls the voice signal input switching unit 150 to switch an input signal from the 2-channel stereo to a 5.1-channel signal.
The voice signal encoding unit 151 encodes a signal in an MPEG-2 AAC system. For the 5.1 channel, the “3/2+LFE” is indicated by an MPEG-2 ADTS fixed header and also a downmixing coefficient is transferred by a PCE (Program Configuration Element). These information are contained in a voice signal stream.
FIG. 15 illustrates a receiving device 1500 for receiving the 5.1-channel surround broadcast. The receiving device 1500 includes an antenna 101, a demodulation unit 102, a demultiplexing unit 103, a packet analysis unit 110, a stream information analysis unit 111, an AAC 2-channel decoder 112, an AAC 5.1-channel decoder 113, a downmixing coefficient analysis unit 114, a downmixing synthesis unit 115, a packet analysis unit 125, and a selector 116.
Since voice reproduction of a typical TV receiver is usually performed through the 2-channel stereo, the receiving device 1500 is configured such that after once performing decoding processing on the 5.1 channel surround broadcast, downmixing to the 2-channel stereo signal is performed.
The demodulation unit 102 performs demodulation on broadcast waves received from the antenna 101 to reproduce a transport stream. The transport stream is forwarded to the demultiplexing unit 103. The demultiplexing unit 103 performs segmentation on the transport stream and extracts PES data and Section data from the transport stream. The section data is analyzed in the packet analysis unit 125 to extract PAT/PMT, which is used as, for example, program information. The PES data is analyzed in the packet analysis unit 110 to extract the selected stream.
The stream analyzed and selected in the packet analysis unit 110 is further analyzed in the stream information analysis unit 111 to perform segmentation to an AAC header, a basic signal, and others. If the header includes an ID for the 2-channel stereo, the basic signal is subjected to decoding processing into a 2-channel stereo signal in the AAC 2-channel decoder 112 and forwarded to selector 116 to be output as the 2-channel stereo signal.
If the header includes an ID for the 5.1 channel surround, the basic signal is subjected to decoding processing into a 5.1-channel signal in the AAC 5.1-channel decoder 113. The decoded 5.1 channel signal is then downmixed from the 5.1 channel to the 2 channel in the downmixing synthesis unit 115. A downmixing coefficient required for the downmixing at the downmixing synthesis unit 115 may be retrieved from the PCE of a stream header is used. The 2-channel stereo signal subjected to the decoding processing and downmixing in is selected by the selector 116 and outputted as a 2-channel stereo signal.
As noted above, to reproduce the 5.1 channel signal, the receiving device 1500 first performs decoding on the 5.1 channel and then performs downmixing to convert the decoded 5.1 channel signal into a 2-channel signal. As a result, the receiving device 1500 may increase the processing volume and may reduce power saving.
Therefore, there is a need for a system that allows multichannel reproduction and reduces the delay in reproducing the voice signal when the format of the voice signal changes from one channel to another (e.g., from 2-channel stereo to 5.1 channel surround signal).